The present invention relates to a procedure by which the transmission and reception frequencies required in a digital radio telephone based on time division multiplex access (TDMA), in particular, in a radio telephone for the Group Special Mobile (GSM) system can be formed.
As is known in the art, generally at least two frequency synthesizers are used to generate the frequencies required by the transmitter and the receiver of a radio telephone. These synthesizers generate stable local oscillator frequencies for the modulators of the transmitter and for the demodulators of the receiver. In these known radio telephones different oscillators are also used for forming the clock frequencies for the logic of the telephone and the reference frequency for the radio frequency signal. This arrangement is appropriate for use also in TDMA systems, for instance in the completely digital GSM system covering all Europe and scheduled to be adopted into use in the middle of the year 1991.
In analog radio telephones, e.g. in 900 MHz systems, it is well known in the art to use one synthesizer to form a mixer frequency, for transferring a received RF signal received thereby being transmitted to an intermediate frequency, and a mixer frequency, whereby the intermediate frequency of the transmitter is raised to the transmission frequency.
The frequencies of the above systems are e.g. as follows.
______________________________________ Receiver: receiving frequency 935 to 960 MHz intermediate frequency 50 MHz mixer frequency 985 to 1010 MHz (local oscillator) Transmitter: transmission frequency 890 to 915 MHz intermediate frequency 95 MHz mixer frequency 985 to 1010 MHz (local oscillator) ______________________________________
As is noted, the difference between the intermediate frequencies, i.e. 45 MHz, is the same as the duplex interval of the system. Therefore, it is feasible to use only one synthesizer, whereby the mixer frequency generated is the same both for the transmitter and for the receiver. If the difference between the mixer frequencies were different from the duplex intervals, at least two synthesizers should be used. In a digital radio telephone in which the difference between the intermediate frequencies differs from the duplex interval, e.g. by 52 MHz, at least two synthesizers should be used for forming intermediate frequencies.
Using several synthesizers and oscillators in a digital radio telephone no doubt raises the manufacturing costs, requires more space on circuit boards, and increases the failure occurrence rate of the telephone.